Protective film for polarizing plate

Abstract

A protective film for polarizing plate comprising k layers (k is an integer of 2 or more) of thermoplastic resin layer laminated, wherein a refractive index n i (380) at a wavelength of 380 nm and a refractive index n i (780) at a wavelength of 780 nm in the i th thermoplastic resin layer as well as a refractive index n i+1 (380) at a wavelength of 380 nm and a refractive index n i+1 (780) at a wavelength of 780 nm in the i+1 th thermoplastic resin layer have a relationship in ∥ n i (380)− n i+1 (380)|−| n i (780)− n i+1 (780)∥≦0.02 (where, i is an integer of 1 to k−1). A polarizing plate is obtained by bonding the protective film for the polarizing plate and a polarizer. And a liquid crystal display comprising the polarizing plate and a liquid crystal panel.

1. A protective film for a polarizing plate comprising k layers (k is an integer of 2 or more) of thermoplastic resin laminated, wherein
 the protective film is obtained by coextrusion, and 
 the refractive index n i (380) at the wavelength of 380 nm and refractive index n i (780) at the wavelength of 780 nm in the i th thermoplastic resin layer as well as refractive index n i+1 (380) at the wavelength of 380 nm and refractive index n i+1 (780) at the wavelength of 780 nm in the i+1 th thermoplastic resin layer have a relationship in Formula [1], wherein the i th thermoplastic resin is different from the i+1 th thermoplastic resin,
   ∥ n   i (380)− n   i+1 (380)|−| n   i (780)− n   i+1 (780)∥≦0.02  Formula [1]
 
 
 
       (i is an integer from 1 to k−1). 
     
     
       2. The protective film for the polarizing plate according to  claim 1 , wherein refractive index n i (λ) at a wavelength λ in the range from 380 to 780 nm in the i th thermoplastic resin layer and refractive index n i+1 (λ) at a wavelength λ in the range of 380 to 780 nm in the i+1 th thermoplastic resin layer have the relationship in Formula [2] and the absolute value of photoelastic coefficient is 10×10 −12 Pa −1  or less,
   | n   i (λ)− n   i+1 (λ)|≦0.05  Formula [2]
 
 
       (i is an integer from 1 to k−1). 
     
     
       3. The protective film for the polarizing plate according to  claim 1 , wherein every one of the k layers of thermoplastic resin is made of a material with a haze of 0.5% or less and containing an amorphous thermoplastic resin, and
 the humidity expansion coefficient β i  of the i th thermoplastic resin layer and the humidity expansion coefficient β i+1  of the i+1 th thermoplastic resin layer have the relationship in Formula [3],
   |β i −β i+1 |≦40 ppm/% RH  Formula [3]
 
 
 
       (i is an integer from 1 to k−1). 
     
     
       4. The protective film for the polarizing plate according to  claim 1 , wherein at least one of the k layers of thermoplastic resin is a thermoplastic resin layer with the water absorption coefficient of 0.5% or less. 
     
     
       5. The protective film for the polarizing plate according to  claim 1 , in which at least one layer is composed of a thermoplastic resin having a negative photoelastic coefficient, and at least one layer is composed of a thermoplastic resin having a positive photoelastic coefficient. 
     
     
       6. The protective film for the polarizing plate according to  claim 1 , in which the thermoplastic resin constituting the protective film is polystyrene, poly-α-methylstyrene, polysulphone resin, polycarbonate resin, polyethersulphone resin, polyethylene terephthalate resin, polyimide resin, polymethylmethacrylate resin, polyarylate resin, polyethylene resin, polyvinyl chloride resin, cellulose diacetate, cellulose triacetate, or alicyclic olefin polymer. 
     
     
       7. A polarizing plate comprising a laminate of the protective film for polarizing plate according to  claim 1  and a polarizer. 
     
     
       8. The polarizing plate according to  claim 7 , wherein the polarizer contains polyvinyl alcohol;
 said protective film for polarizing plate is laminated on the polarizer facing to the first thermoplastic resin layer therein; and 
 refractive index n 1 (380) at the wavelength of 380 nm and refractive index n 1 (780) at the wavelength of 780 nm in the first thermoplastic resin layer as well as refractive index n b (380) at the wavelength of 380 nm and refractive index n b (780) at the wavelength of 780 nm in the polyvinyl alcohol have the relationship in Formula [4],
   ∥ n   1 (380)− n   b (380)|−| n   1 (780)− n   b (780)∥≦0.02  Formula [4].
 
 
 
     
     
       9. The polarizing plate according to  claim 7 , wherein refractive index n 1 (λ) at the wavelength λ in the range of 380 to 780 nm in the first thermoplastic resin layer in said protective film for the polarizing plate and refractive index n b (A) at the wavelength λ in the range of 380 to 780 nm in the polyvinyl alcohol contained in said polarizer satisfy the relationship in Formula [5],
   | n   1 (λ)− n   b (λ)|≦0.04  Formula [5].
 
 
     
     
       10. The polarizing plate according to  claim 7 , wherein there is no linear recess portion with a depth of 50 nm or more and a width of 500 nm or less on the surface of the k th thermoplastic resin layer in said protective film for the polarizing plate. 
     
     
       11. The polarizing plate according to  claim 7 , wherein there is no linear projection portion with a height of 50 nm or more and a width of 500 nm or less on the surface of the k th thermoplastic resin layer in said protective film for the polarizing plate. 
     
     
       12. A liquid crystal display comprising the polarizing plate according to  claim 7  and a liquid crystal panel. 
     
     
       13. The liquid crystal display according to  claim 12 , wherein said polarizing plate is arranged on the observer side of a liquid crystal panel.